The present invention pertains to reclaiming foundry sand, be it green sand or sand used in molded cores for reuse or safe disposal.
In the production of certain types of metal castings, large or small, e.g. aluminum, iron or steel, the casting mold is prepared by the application of suitable binders or adhesives to specifically sized aggregates such as silica sand, specialty sands or synthetic sands. The adhesives most commonly used include natural clays activated by water and inorganic and organic resins cured by various catalysts, such as acids, bases or heat activation. In the founder's lexicon, the term "green sand" refers to sand that is bonded with a mixture of clay and water. Water is added in specified amounts to activate the fine ground clay which has been mixed with the specially prepared aggregate, sand. This homogeneous mixture of sand which has been coated with water activated clay is then applied to patterns using pressure, vibration or other means of compaction to form the container or "mold" into which molten metal is poured to form the casting.
Alternatively to clay/water adhesives, the use of synthetic organic and inorganic resins are commonly used to prepare molds capable of withstanding the regors of the metal casting process. In the preparation of resin bonded sand molds, washed and dried aggregates such as silica sand, lake sand, synthetic aggregates, specialty sands such as olivene, chromite and Zircon sands are mixed with resins in mullers, batch mixers or continuous mixers to coat the aggregate particles with the resins. Curing or hardening of the resin films or adhesives which coat the sand grains can be achieved in a wide variety of methods including, catalysis, heat, or through the use of gases or vapors. Some resin systems employed can also be autocatalytic or self setting.
The terminology "green sand" describes the natural state of clay/water activated adhesives since it is similar to green ware in ceramics or wood, where the term green means that the ceramic has not been fired or dried in a kiln or oven. In the case of wood, the wood has not been subjected to a drying operation to reduce its moisture content. In addition to sand, the aggregates, which can be silica, zircon, chromite, olivine, ceramic or synthetic, and the clay binder, which can be western bentonite, southern bentonite or other clays such as fire clay, the foundry sand may also contain additives such as cereal, in the form of corn, milo, wheat and rye flours, cellulose in the form of finely ground wood flour, oat hulls, rice hulls and ground nut shells, carbon in the form of seacoal, (low sulfur coal), gilsonite, lignite and polymers or chemicals, such as water, or polymers, wetting agents, soda ash and iron oxide to name a few.
The foundry process also includes the use of bonded aggregates to produce cores or shaped sand necessary to form the internal passages or surfaces. The same sand that is used to make the mold can also be used to make cores which are placed in the mold to achieve hollows, slots, passages, holes and the like in the finished castings. Cores are generally made from new sand since the presence of contaminates such as clays, fines, water or organic and inorganic materials interfere with the adhesives bonding mechanism chemically or physically. Synthetic sands may also be employed to impart special characteristics to the cores when they are exposed to the casting process. Again, as in the production of resin bonded molds, adhesives or resins are coated on washed and dried specifically sized aggregates which are cured through a variety of methods described above for molding with resin systems. Examples of no bake binders are furan and phenolic/acid cured systems, phenolic/ester cured systems, alkyd oil urethanes, alumina phosphate, and silicate/ester mixtures. Examples of cold box binders are acrylic epoxy SO2, (free radical or acid cured), furan SO2, phenolic urethane amine cured systems, ester cured alkaline phenolics, sodium silicate CO2 and phenolic CO2 cured systems. Examples of heat cured binders are hot box-furan and phenolic resins, warm box-furan and phenolic, shell, core oil and aluminate silicates.
In the manufacture of castings, after the molten metal is poured into the mold and solidification has occurred, the mold is subjected to "shake-out". Shake-out refers to the separation of the sand from the casting(s). The casting is then sent to various finishing operations and the sand is subject to either reclamation, reuse or disposal.
The most prevalent foundry molding method used is the green sand process followed by chemically bonded no-bake molding. Green sand molding without insertion or use of cores allows the mixture of sand, cereal, clay, water, seacoal, etc. to be reactivated through the addition of new clay, water and additives in mixers or mullers. However, new sand must be added to replace the sand lost in the casting process since handling, high temperatures and fracturing of the sand can occur.
In the case of castings which have internal passages or those which are hollow, the use of cores adds sand to the system or green sand which dilutes the clay bonded sand. Again, additions of clay, water, seacoal etc. must be made to maintain the desired properties of the green sand system.
Since most castings made in green sand systems and no-bake or chemically bonded molding require cores, the ability to reclaim the used or spent sand would be extremely desirable. In the past, disposal of foundry sand in a landfill site was one way of disposing of the sand after the shake-out operation. However, because of the ever changing environmental rules and regulations and the increasing costs for acquisition, preparation and delivery of new sand, efforts have been focused on the reclamation and reuse of sand and aggregates used in the casting process.
Attempts to reclaim sand for use within the foundry have not been successful for a variety of reasons. While green sand can be reprocessed for re-use in clay bonded molding sand, the reclamation of clay bonded sand has not been successful for a variety of physical and chemical reasons. These include alterations to grain fineness number, particle size distribution, contamination, moisture, changes in pH or acid demand value, and surface area changes to name a few.
Attempts to reclaim bentonite or clay bonded systems have included attrition, washing and thermal treatment. The most prevalent method of reclaiming sand values from foundry sand is through the application of mechanical treatment, thermal treatment or combinations of both. Thermal units typically employ infrared or gas fired thermal sources. In the traditional process for green sand reclamation, the ionic bond of clay systems is deactivated by calcination of the clay. The calcined clay, known as dead clay, can then be stripped from the sand by mechanical means, e.g. by high energy pneumatic stripping which impacts a stream of sand on a target and mechanically blasts the clay particles from the sand grain, or by imparting energy in the form of attrition, scrubbing or subjecting the particles to mechanical treatment.
Physical abrasion of the agglomerated and individual sand grains does not remove all of the adhesives from the sand particles since the irregular shapes on the sand surface do not always unlock the entrapped clay or resin particles. This, combined with the fact that the mechanical stripping results in a change in the particle size distribution of the sand so that the particle size distribution must be readjusted with the addition of new sand additions to maintain the desired size distribution. Too fine or coarse particle distribution results in inferior molding properties and can produce adverse affects upon the castings produced, such as, gas related and metal penetration defects.
Thermal reclamation of green sand or resin bonded sands typically operate at temperatures in excess of 1600 degrees F. (871.degree. C.) for bentonite bonded and inorganic bonded sands and in excess of 900.degree. F. for organic based adhesive systems. The process of thermal reclamation includes both heating and cooling followed by mechanical stripping, sand cooling and classification of the sand for reblending or rebonding. The overall process can result in a sand fraction that may not meet original specifications and a waste stream of silica fines and dead clay, all of which must be disposed of in a landfill or by other environmentally acceptable means.
A second type of reclamation is the use of mechanical attrition to mechanically breakdown the lumps or agglomerated sand particles into individual sand grains when resins or adhesives are used in place of clay bonded systems. Although mechanically reclaimed sand can be used in most chemically bonded systems, the returned or reclaimed sand typically contains residues of resin and carbonaceous materials which interfere with rebonding of the sand or produce undesirable casting conditions. The presence of residuals not removed by mechanical reclamation increase the fineness of the sand which typically requires greater levels of binder additions to maintain equivalent strength for handling and pouring. In addition, the higher levels of adhesives in the system can contribute to casting defects.
In a thermal process it is typical that about 1 million Btu's of energy be consumed per ton of reclaimed sand. In addition to the heat energy, energy must be expended to cool and classify the sand as well as to provide for whatever environmental regulations require. In many instances, thermally treated sand may require additions of chemicals to alter the pH and acid demand value of the sand to make it suitable for reuse in the core production area or in chemically bonded systems.
Thermal processes work well on most chemically bonded sands, but as stated above, do not work as well on clay bonded systems. Numerous schemes have been used to provide exposure of the sand to the source of heat, such as rotary kilns, fluidized beds and mechanical stirring. All of the thermal reclamation systems are sensitive to sand composition, binders and the amount of metallic oxides present in the sand, regardless of how the sand is heated. Thermal reclamation units require periodic relining and extensive environmental regulations govern their use. For example, calciners have been classified as fluid bed incinerators rather than reclaimers, thus requiring the operators to respond to different and more stringent environmental rules and regulations. It is estimated that, on average, to construct and verify operability of a thermal reclamation system will cost an operator about 500 thousand dollars per ton of capacity per hour of operation.
Additional discussions of foundry sands, binder systems and additives can be found in a series of papers published in AFS Transactions of the American Foundry Society. These are "If it's Black, Why do they scall it Green Sand" by D. F. Hoyt, AFS Transactions 1995, Vol. 103, Pages 353-369 (#95-100), "Scanning Electron Microscope and Sand-Binder Studies: A 25-Year Review" by R. H. Toeniskoetter, AFS Transactions 1995, Vol. 103, Pages 477-486 (#95-144), "Sand Reclamation Project: Saginaw Malleable Iron Plant, GM Powertrain Group" by D. J. Couture, R. L. Havercroft and L. L. Stahl, AFS Transactions 1995, Pages 783-790 (#95-141), "Evaluation of Reclaimed Green Sand for Use in Various Core Processes" by S. E. Clark, C. W. Thoman, R. H. Sheppard, R. Williams and M. B. Krysiak, AFS Transactions 1994 Vol. 102, Pages 1-12 (#94-02) and "Thermal Reclamation The Evidence Against It" by D. S. Leidel, AFS Transactions 1994, Vol 102, Pages 443-453 (94-10).
Ashland Chemical Company has collected thirteen additional papers in a re-print publication titled Sand Binder Systems under the cover Foundry Management & Technology (1996).
Therefore, there is a need for yet another method of reclaiming foundry sand.